1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and, more particularly, to a nonaqueous electrolyte secondary battery containing an improved a nonaqueous electrolyte.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte battery has attracted attention as a high energy density battery. Of such nonaqueous electrolyte batteries, a primary battery using a light metal such as lithium, sodium, or aluminum as a negative electrode active material and manganese dioxide (MnO.sub.2), carbon fluoride [(CF)n], thionyl chloride (SOCl.sub.2), or the like as a positive electrode active material is already widely used as a power source of a timepiece or a backup battery of a memory.
In addition, as the sizes and weights of various types of electronic equipment such as communication equipment have been decreased, a demand for a secondary battery having a high energy density which can be suitably used as a power source of such equipment has been increased, and a nonaqueous electrolyte secondary battery has been actively studied. For example, a nonaqueous electrolyte secondary battery using lithium as a negative electrode and an electrolyte prepared by dissolving an electrolytic salt such as LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, or LiPF.sub.6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL), or tetrahydrofuran (THF) has been studied. In addition, a compound which topochemically reacts with lithium such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, or V.sub.6 O.sub.13 has been studied as a positive electrode material.
The above secondary battery, however, has not been put into practical use yet. This is mainly because a charge/discharge efficiency of the battery is low and its number of charge/discharge times or cycle life is short. The reason for this is assumed that lithium as a negative electrode is degraded due to a reaction with an electrolyte. That is, lithium dissolved in an electrolyte as lithium ions upon discharge reacts with a solvent and its surface is partially deactivated when it precipitates upon charge. Therefore, when charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from a collector.
On the other hand, nonaqueous electrolyte secondary batteries realized by utilizing a carbonaceous material that can absorbed and discharge lithium to be used as an active material of a negative electrode are disclosed in U.S. Pat. Nos. 4,668,595 and 4,702,977. The U.S. Pat. No. 4,668,595 also describes the use of tetrahydrofuran, .gamma.-butyrolactone, dimethoxyethane, propylene carbonate, ethylene carbonate or a mixture thereof as a nonaqueous solvent for preparing a nonaqueous electrolyte.